The invention relates to the field of in situ detection of subsurface contamination, and more particularly, to a system which detects subsurface fissionable nuclear contamination by means of photoneutron emissions and detection.
The purpose of this invention is to provide a cost effective means for in situ detection of fissionable materials such as uranium and plutonium which may be present as soil contaminants in the vicinity of nuclear material processing facilities. Previous methods for detecting such contamination require that samples be extracted and sent for laboratory analysis. This procedure involves delay and labor intensive handling. A far more desirable approach is to detect and quantify fissionable contamination in situ. Such an approach allows for the possibility of real time three dimensional mapping of the contaminants.
Passive detection of the natural radioactive emissions from fissionable materials is limited by the relatively low level of such emissions and the relatively high background level associated with natural terrestrial sources. An alternative approach is to selectively increase the radiation emission rate of the contaminants by active stimulation. An example of active stimulation of radiation emission is photonuclear stimulated neutron emission or photoneutron generation.
Photoneutrons are generated when an energetic photon interacts with a nucleus. If the energy of the photon exceeds the threshold binding energy of neutrons in the nucleus, a photoneutron can be liberated from the nucleus. The energy threshold for this process is dependent on the isotopic species of the nucleus with which the photon interacts. By observing the threshold at which photoneutron generation occurs it is possible to detect and distinguish the presence of specific elements. This technique is useful for fissionable contaminants such as uranium and plutonium, and also for other selected contaminants such as beryllium.
Photoneutron stimulation is not commonly used as an analytic technique because the photon energy required is very high. The photon energy threshold ranges from 1.665 MeV for beryllium to in excess of 7 MeV for many common metals and rare earth elements. The threshold energy range for fissionable nuclear contaminants is between approximately 3 to 7 MeV. Operation of sources which generate such energetic photons involves significant safety issues, and for laboratory analysis many alternative methods are available, including many techniques of chemical analysis and spectroscopy of alpha particle and gamma radiations.
For in situ analysis of soil contamination the laboratory techniques are not generally practical, since laboratory instruments are not designed to be operated inside holes drilled in the ground. Therefore, the current practice is to remove samples as noted above.
Therefore, there is a need for an apparatus and method for in situ analysis of fissionable nuclear contamination in subsurface soil.